I think what OP's really asking is why you would deliberately only use 2 ammo on the pull when you have the capability to stack up to 3 of them (be it loading only 2 ammo, or using reload then overwriting it with another reload even though it has 1 ammo left). Like, why does a '2 ammo only' opener even exist? The answer likely has to do with how that sets up your ammo cooldown usage for the rest of the fight, as well as your heat gain per 60s cycle. I've been bard for this whole expansion so maybe someone else can fill him in.

It can be a bit counterintuitive because it seems like you're throwing away something for free for no good reason.